1. Field of the Invention
This invention relates to a gas sensor of thick film type and to a method of producing a gas sensor having a substrate and a thick film sensor element firmly bonded to the substrate.
Application of the thick film techniques has been expanding recently with the progress of the so-called hybrid techniques. For instance, the thick film techniques are applied to production of a sensor by forming a metal oxide thick film on a ceramics substrate, which metal oxide has a temperature-sensitive or gas concentration sensitive resistance; production of a ceramic capacitor by forming a metal oxide thick film on a ceramics substrate, which metal oxide has a high dielectric constant; formation of a protective thick film such as alumina (Al.sub.2 O.sub.3) thick film on a substrate of silicon nitride (Si.sub.3 N.sub.4) or the like for improving the corrosion-resistivity or acid-resistivity of the substrate.
The invention provides a remarkable improvement in the bond between an insulating ceramics substrate and a thick film formed thereon, which improvement is applicable to various kinds of thick films.
2. Description of the Prior Art
FIG. 1 shows a typical conventional thick film gas sensor element formed on a substrate. More specifically, an insulating ceramics substrate 1 carries electrodes 2a and 2b of a certain desired configuration formed thereon by printing. A sensor element 3 is formed on the substrate 1 by printing a thick film of paste thereon, which paste mainly consists of gas-sensitive metal oxide, and firing the thus printed thick film of paste. The sensor thus formed has various advantages; for instance, the sensor element 3 is thin and has excellent response characteristics, and a heater indispensable to many sensors can be formed on the same substrate simply by printing a thick film heater resistor thereon.
The above sensor, however, has a disadvantage in that the bond between the substrate 1 and the sensor element 3 is comparatively weak, because the top surface 1a of the above substrate 1 to which the sensor element 3 is fired is flat and smooth as shown in the sectional view of FIG. 2 taken along the line II--II of FIG. 1. When the gas sensor is exposed to frequent temperature change over a wide range such as that experienced in automobile engine exhaust gas, thermal strain is caused in the sensor element 3 due to the difference of coefficient of thermal expansion between the substrate 1 and the sensor element 3, and such thermal strain tends to cause separation of the sensor element 3 from the substrate 1. To reduce the risk of such separation, the substrate 1 may be made by using material consisting of ceramics particles with a comparatively large diameter, so as to make the surface thereof uneven. On the other hand, the use of large ceramics particles is apt to deteriorate the properties of the substrate 1, such as electric insulation, mechanical strength, and the like.
It has been proposed to use a bond-improving additive, such as glass, in the material of the thick film of certain kind by D. I. Herbst et al in "Int. Microelectronics Symp. (1971) 4.7" and by M. V. Coleman et al in "Proc. Inst. Electro. Radio Eng. Conf. (1976) [31] 1-16". However, the use of an additive often results in inevitable change of the properties of the thick film itself.
As another method for improving the bond, W. H. Kohl suggested to select such combination of a substrate and a thick film wherein they have similar coefficients of thermal expansion, in the book entitled "Materials and Techniques for Vacuum Devices" (1967) Reinhold Pub. at page 391. This combination is effective in reducing the thermal strain and preventing the separation even if the adhesion between the thick film and the substrate is not so strong. However, this combination does not provide sufficient impact strength.
Further, it has been repeatedly attempted to improve the bond of the thick film by roughening the surface of the substrate therefor; for instance, by grinding the substrate surface by grains of large diameter or by shot blasting, as well known in the art. It has been also attempted to cover the substrate surface with crystalline particles of large diameter, so as to make use of projections formed thereby, but a satisfactory bond was not obtained due to the difference in the sintering characteristics between the crystalline particles and the substrate caused by the difference in crystal grain size therebetween. Besides, in the covering with the crystalline particles, it was impossible to provide a homogeneously controlled unevenness. In general, the conventional roughening of the substrate surface did not provide sufficient bonding strength for films thicker than 10 .mu.m. The above grinding or shot blasting for roughening the substrate surface hurts the substrate in the end, so that its tends to weaken the strength of the substrate.
In short, there has not been any method suitable for improving the bond of a thick film, especially a functional semiconductor thick film, with a substrate therefor.